Electrodeionization apparatus with fixed ion exchange materials

ABSTRACT

Electrodeionization apparatus for purifying water that includes a cathode, an anode, and a plurality of alternating anion permeable membranes and cation permeable membranes between the cathode and anode that define concentrating and diluting flow channels between adjacent pairs of membranes. The diluting channels include cation exchange materials and anion exchange materials that are fixed in close contacting position with respect to each other and provide conductive paths for cations and anions to the adjacent membranes and provide flow passages for water between the materials. The anion exchange materials and cation exchange materials each have a characteristic dimension that is smaller than the characteristic dimensions of the flow passages. The use of exchange materials with small dimensions and the fixed intimate contact of cation and anion exchange materials provides increased, uniform water splitting and resin regeneration, and a high rate of ion removal from the water flowing through the diluting channels compartments. An increased velocity can be provided in the diluting channels of by reintroducing a portion of the water from the diluting channel outlet to the diluting channel inlet or using flow diverters in the diluting channel to provide a tortuous path for the flowing water.

BACKGROUND OF THE INVENTION

The invention relates to apparatus and methods for carrying outelectrodeionization to purify water.

Electrodeionization is a process for removing ions from liquids bysorption of these ions into a solid material capable of exchanging theseions for either hydrogen ions (for cations) or hydroxide ions (foranions) and simultaneous or later removal of the sorbed ions intoadjacent compartments by the application of an electric field. (SeeGlueckauf, E., “Electro-Deionization Through a Packed Bed”, December1959, pp. 646-651, British Chemical Engineering for a backgrounddiscussion.) The hydrogen and hydroxide ions needed to drive the ionexchange process are created by splitting of water molecules at theinterface of anion and cation exchanging solids which contact each otherin the orientation that depletes the contact zone of ions, when in thepresence of an electric field. This orientation requires that the anionexchanging material face the anode and the cation exchanging materialface the cathode. The created hydroxide ions enter the anion exchangingmaterial, and the created hydrogen ions enter the cation exchangingmaterial.

The electrodeionization process is commonly carried out in an apparatusconsisting of alternating diluting compartments and concentratingcompartments separated by anion permeable and cation permeablemembranes. The diluting compartments are filled with porous ionexchanging solid materials through which the water to be deionizedflows. The ion exchanging materials are commonly mixtures of cationexchanging resins and anion exchanging resins (e.g., U.S. Pat. No.4,632,745), but alternating layers of these resins have also beendescribed (e.g., U.S. Pat. Nos. 5,858,191 and 5,308,467). Ion exchangingmaterials consisting of woven and non-woven fibers have also beendescribed. E.g., U.S. Pat. No. 5,308,467 describes a fabric in whichbundles of cation-exchange fibers and are woven alternately with bundlesof anion-exchange fibers, and U.S. Pat. No. 5,512,173 describes a clothcontaining cation exchange fibers, anion exchange fibers and ionicallyinactive fibers. The compartments adjoining the diluting compartmentinto which the ions are moved by the applied electric field, calledconcentrating compartments, may be filled with ion exchanging materialsor with inert liquid permeable materials. An assembly of one or morepairs of diluting and concentrating compartments, referred to as a “cellpair”, is bounded on either side by an anode and a cathode which applyan electric field perpendicular to the general direction of liquid flow.

The diluting compartments are each bounded on the anode side by an anionpermeable membrane and on the cathode side by a cation permeablemembrane. The adjacent concentrating compartments are eachcorrespondingly bounded by a cation permeable membrane on the anode sideand an anion permeable membrane on the cathode side. The appliedelectric field causes anions to move from the diluting compartmentacross the anion permeable membrane into the concentrating compartmentnearer the anode and cations to move from the diluting compartmentacross the cation permeable membrane into the concentrating compartmentnearer the cathode. The anions and cations become trapped in theconcentrating compartments because the movement of anions toward theanode is blocked by a cation permeable membrane, and the movement ofcations toward the cathode is blocked by an anion permeable membrane. Aflow of water is set up to remove the ions from the concentratingcompartments. The net result of the process is the removal of ions fromthe water stream flowing through the diluting compartments and theirconcentration in the water flowing through the concentratingcompartments.

The removal of the ions from the diluting compartment is a multi-stepprocess involving diffusive steps as well as electrically driven steps.First, it is clear that the movement of ions directly from the dilutingsolution across the bounding membranes, under the influence of theapplied electric field, contributes insignificantly to the overallremoval of these ions, because the concentration of ions in the dilutingsolution is typically 1,000 to 100,000 times smaller than theconcentration of ions in the solid ion exchanging materials. While themobility of ions in the solid material may be on the order of 20 timessmaller than their mobility in the solution, the electric field actingon the ions in the two phases is the same, so the product of mobilitytimes concentration times electric field strength, which determines therate of ion removal, is 50 to 5,000 times as large in the solid ionexchanging material.

Glueckauf showed that the mechanism of ion removal from the dilutingcompartment solution includes two steps. The first step is the diffusionof cations to the cation exchanging solids and the diffusion of anionsto the anion exchanging solids. The second step is electrical conductionwithin the solids phases to the bounding membranes of the dilutingcompartment. Because the concentration of ions in ion exchanging solidsis so high, the process that controls the overall removal of ions istheir rate of diffusion from the solution to the surface of the ionexchanging solids. This diffusion rate is a function of three factors;the diffusion rate is proportional to surface area between the ionexchanging solids and the flowing solution, inversely proportional tothe thickness of the liquid layer through which the ions must diffuse,and proportional to the difference in concentration of the ions in thebulk of the diluting solution and their concentration next to the ionexchanging solid. In order to achieve high rates of ion removal, theproduct of the above three factors should thus be as high as possible.The ratio of the surface area to the diffusion distance is inverselyproportional to the characteristic dimension of the ion exchanging solidmaterial; the characteristic dimension is particle radius for ionexchange resins and is fiber diameter for ion exchange fibers. Indesigning electrodeionization apparatus, this characteristic dimensioncan be made as small as possible, commensurate with avoidance ofexcessive pressure drops or plugging by particles in the water to betreated. Particle diameters on the order of 500 to 600 micrometers aretypical, and fiber diameters can be on the order of several tens ofmicrons.

As noted above, the third factor controlling the rate of ion removal isthe difference in concentration of the ion being removed between thebulk of the solution and its concentration in the liquid adjacent to thesurface of the ion exchanging solid where it is being exchanged foreither a hydrogen or a hydroxide ion. The concentration of the ion inquestion at the surface of the ion exchanging solid is in equilibriumwith the concentration of that ion in the solid. For cations, theequilibrium concentration is approximately equal to the ratio of thecation concentration to the hydrogen ion concentration in the cationexchanging solid times the concentration of the cation in solution. Foranions, the equilibrium concentration is approximately equal to theratio of the anion concentration to the hydroxide ion concentration inthe anion exchanging solid times the concentration of the anion insolution. In order for this equilibrium concentration to be low, and therate controlling concentration difference to be large, the cationexchanging solid should be predominantly in the hydrogen form, and theanion exchanging solid should be predominantly in the hydroxide form. Infact, if the two solids are completely in the ionic form rather than inthe hydrogen or hydroxide form, there is no concentration difference,and ions will not be removed by this diffusive mechanism.

In order for the ion exchanging solids to be predominantly in thehydrogen and hydroxide forms, the so-called “regenerated forms,” therate of hydrogen ion and hydroxide ion creation (water splitting) mustbe both high and spatially uniform. A high average rate of watersplitting can be achieved by applying a high voltage drop across thediluting compartment. With equinormal mixtures of ion exchangeparticles, voltages of between 1 and 5 volts are adequate for thepurpose. The achievement of a uniform distribution of water splitting isa more difficult problem and much effort has gone into designingstructures that achieve this (e.g., U.S. Pat. Nos. 5,858,191, 5,868,915and 5,308,467). The random nature of mixtures of cation and anionexchanging particles tends to cause some portion of the particles to beregenerated to a needlessly high degree and others inadequatelyregenerated. Water flowing through the regions of inadequatelyregenerated material will be inadequately purified. The essence of thedifficulty that existing approaches have had in dealing with the problemis that the number of contacts between cation exchanging and anionexchanging material where water splitting can take place is limited bythe relatively large characteristic dimension of the ion exchangingmaterial. This results in regions of inadequately regenerated resinbetween the water splitting sites.

SUMMARY OF THE INVENTION

In one aspect, the invention features, in general, electrodeionizationapparatus for purifying water. The apparatus includes a cathode, ananode, and a plurality of alternating anion permeable membranes andcation permeable membranes between the cathode and anode that defineconcentrating and diluting flow channels between adjacent pairs ofmembranes. The diluting channels include cation exchange materials andanion exchange materials that are fixed in close contacting positionwith respect to each other and provide conductive paths for cations andanions to the adjacent membranes and provide flow passages for waterbetween the materials. The anion exchange materials and cation exchangematerials each have a characteristic dimension that is smaller than thecharacteristic dimensions of the flow passages. The use of exchangematerials with small dimensions and the fixed intimate contact of cationand anion exchange materials provides increased, uniform water splittingand resin regeneration, and a high rate of ion removal from the waterflowing through the diluting channels compartments.

Particular embodiments of the invention may include one or more of thefollowing features. Individual particles of cation exchange material andanion exchange material can be fixed together with a binder insufficient particle concentration to provide conductive paths forcations and anions. The particles of cation exchange material, anionexchange material and binder can form larger combined particles packedinto the diluting flow channel in contacting relation between adjacentmembranes. The combined particles are sufficiently large so as to causean acceptably low pressure drop in the diluting flow channel.Alternatively, the particles of the cation exchange material and theanion exchange material and binder can form filaments provided as amatrix between the adjacent membranes. The openings in the matrix forwater flow are larger than the diameter of the filaments. A furtheralternative is to have particles of cation exchange material, anionexchange material and binder form an open cell foam between adjacentmembranes, with the openings in the foam being sufficiently large toprovide flow passages through the foam with an acceptably low pressuredrop.

The fixed ion exchange material could also be provided as cationexchange filaments and anion exchange filaments that are intimatelycommingled or joined together. When the fixed ion exchange materials arein the form of filaments, they can be provided in multiple filamentbunches or as multiple filament braids. The strands, made of bunches,braids, or individual filaments, can be fixed with respect to otherstrands by providing them as a woven fabric, nonwoven (randomlyoriented) fabric or extruded netting. The fabric could also be providedby extrusion.

Preferably the majority of combined particles have dimensions greaterthan 0.1 mm, and the majority of individual particles of the cation andanion exchange material have dimensions less than 0.01 mm. The combinedparticles preferably are sufficiently large so as to cause an acceptablylow pressure drop (e.g., less than 100 psig) in the diluting flowchannel. The filaments can have diameters between 0.1 mm and 3.0 mm. Thefabric includes groups of generally parallel filaments, with filamentsspaced center-to-center by a distance equal to or greater than thediameter of filaments. The binder used to fix the individual cation andanion particles is preferably a thermoplastic polymer or thermosettingpolymer, but can be any water insoluble bonding material. The cation andanion exchange materials are made of styrenic ion exchange resin,acrylic ion exchange resin, phenolic ion exchange resin, or carbohydrateion exchange resin.

In another aspect, the invention features, in general, obtaining anincreased velocity in the diluting channels of electrodeionizationapparatus by reintroducing a portion of the water from the dilutingchannel outlet to the diluting channel inlet or using flow diverters inthe diluting channel to provide a tortuous path for the flowing water,while keeping the volume of the diluting channel substantiallyunchanged. The diffusion distance is decreased by increasing thevelocity of the water flowing past the ion exchanged particles.

Embodiments of the invention may include one or more of the followingadvantages. A substantially spatially uniform rate of water splitting isachieved in the diluting channels; the uniform rate is conducive to ahigh and uniform degree of resin regeneration and consequently a highrate of ion removal from the water flowing through the dilutingcompartments. The uniform regeneration of the anion resin additionallyfacilitates removal of silica and carbon dioxide. The small size of theion exchanging particles or filaments insures that numerous anduniformly distributed sites for water splitting are created withoutcreating excessive pressure drops, because the dimension of the passagesfor water flow can be made larger, without affecting adversely the watersplitting properties of the material.

Other advantages and features of the invention will be apparent from thefollowing description of particular embodiments thereof and from theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of electrodeionizing apparatus.

FIG. 2 is a diagrammatic plan view of a woven fabric of deionizingmaterial used in a diluting channel of the FIG. 1 apparatus.

FIG. 3 is a diagrammatic plan view of a fabric of extruded netting ofdeionizing material used in a diluting channel of the FIG. 1 apparatus.

FIG. 4 is a diagrammatic plan view of a nonwoven fabric of (randomlyoriented) strands of deionizing material used in a diluting channel ofthe FIG. 1 apparatus.

FIG. 5 is a diagrammatic elevation of a multifilament strand ofdeionizing material useful in the FIG. 2, 3 or 4 fabric.

FIG. 6 is a diagrammatic elevation of a braided strand of multifilamentsof deionizing material useful in the FIG. 2, 3 or 4 fabric.

FIG. 7 is a diagrammatic perspective view of a filament that containscation exchange and anion exchange fibers in a binder and can be used inthe FIG. 2, 3 or 4 fabric.

FIG. 8 is a diagrammatic vertical sectional view showing the wovenfabric of FIG. 2 in a diluting channel of the FIG. 1 device.

FIG. 9 is a diagrammatic perspective view of an open cell foam thatcontains cation exchange and anion exchange materials therein and can beused in a diluting channel of the FIG. 1 apparatus.

FIG. 10 is a diagrammatic elevation of combined ion exchange particlesuseful in the diluting channel of the FIG. 1 apparatus.

FIG. 11 is diagram showing a partial recirculation loop that can be usedwith the FIG. 1 apparatus.

FIG. 12 is a diagrammatic plan view showing the use of flow diverters indiluting channels of the FIG. 1 apparatus.

DETAILED DESCRIPTION OF A PARTICULAR EMBODIMENT

Referring to FIG. 1 electrodeionization apparatus 10 includes cathode12, anode 14 spaced from cathode 12, and a plurality of alternatinganion permeable membranes 16, and cation permeable membranes 18.Diluting channels 20 (“D”) are provided between each pair of an anionpermeable membrane 16 that faces anode 14 and a cation permeablemembrane 18 that faces cathode 12. Concentrating channels 22 (“C”) areprovided between each pair of an anion permeable membrane 16 that facescathode 12 and a cation permeable membrane 18 that faces anode 14.Diluting channels 20 and concentrating channels 22 can be about 3.0 mmthick. Fixed ion exchange materials 24 are located in diluting channels20, and ion exchange materials or other spacers 25 are located inconcentrating channels 22. As discussed in detail below, fixed ionexchange materials 24 can take a variety of forms. Cathode 12, anode 14,membranes 16, 18 and spacer materials 25 can be made of components andmaterials typically used in electrodeionization apparatus, as described,e.g., in the above-referenced patents, which are hereby incorporated byreference. Water flows are provided past cathode 12 and anode 14. As iswell known in the art, the components shown on FIG. 1 are assembledtogether as a stack between the pressure plates held together by boltsor a hydraulic ram or in a housing that contains the components andprovides manifolds to direct the incoming liquid to and the outgoingliquid from diluting channels 20 and concentrating channels 22. Dilutingchannels 20 and concentrating channels 22 are typically between 1.0 mmand 5.0 mm thick, and there typically are 10 to 300 diluting channels.The surface area of each membrane is typically between 0.5 and 5.0square feet.

Fixed ion exchange materials 24 include cation exchange materials andanion exchange materials that are fixed in close contacting positionwith respect to each other. Fixed ion exchange materials 24 can beprovided in strands 26 of combined anion and cation exchange materialsin woven fabric 28 (FIG. 2), extruded netting fabric 30 (FIG. 3) andnonwoven fabric 32 of randomly oriented strands 26 (FIG. 4). Fixed ionexchange materials could also be provided by open cell foam 50 (FIG. 9)and by combined exchange particles 60 (FIG. 10).

Strands 26 (FIGS. 2-4) can also take a variety of forms. Strand 26 canbe made in the form of bundle 34 of multiple filaments 36, as shown inFIG. 5. Strand 26 can also be in the form of braided strand 38, as shownin FIG. 6; braid 38 is made on a standard braiding machine. Strand 26can also be in the form of combined exchange particle filament 40, whichis made of cation exchange particles 42 (shown white on FIG. 7) andanion exchange particles 44 (shown dark on FIG. 7) that are heldtogether by binder 46. Filaments 36, used in bundle 34 (FIG. 5), couldbe made of roughly equal, commingled amounts of individual filaments ofcation exchange material and individual filaments of anion exchangematerial. Alternatively, combined exchange particle filaments 40 (FIG.7), each having cation exchange particles and anion exchange particles,could be used as filaments 36 in bundle 34. Combined exchange particlefilaments 40 could similarly be used in making braid 38, using either asingle filament 40 or a plurality of filaments 40 in each braidedtogether portion 48 of braid 38. Each portion 48 of braid 38 could alsobe made of a plurality of commingled filaments of cation exchangematerial and filaments of anion exchange material.

Referring to FIG. 9, fixed ion exchange materials 24 could also beprovided as open cell foam 50, which (like filaments 40), includescation exchange particles 52, anion exchange particles 54 and binder 56.Open cell foam 50 has an interconnected network of flow passages 58therethrough.

Referring to FIG. 10, fixed ion exchange materials 24 could also beprovided as combined particles 60, made up of cation exchange particles62 (shown white), anion exchange particles 64 (shown dark) and binder66. Combined particles 60 are sufficiently large so as to cause anacceptably low pressure drop in diluting flow channels 20 in the spacebetween combined particles 60.

Individual cation exchange particles 42, 52 and 62 and anion exchangeparticles 44, 54 and 64 in filament 40 (FIG. 7), foam 50 (FIG. 9), andcombined particle 60 (FIG. 10), respectively, have dimensions (roughly adiameter) of less than 0.1 mm, preferably less than 0.05 mm. Individualfilaments 36 in bundle 34 (FIG. 5) and in braided strand 38 are between0.01 mm and 1.0 mm in diameter. Strands 26, bundles 34, braid 38,combined exchange particle filament 40 and combined particle 60 havediameters between 0.1 mm and 3.0 mm. Particles 42, 44, 62, and 64 arepreferably less than ⅓ the diameter of combined exchange particlefilament 40 or combined particle 60, respectively.

Particles 42, 44, 52, 54, 62, 64 are provided in sufficient particleconcentration to provide conductive paths for cations and anions throughthe bulk filament, foam, or combined particle structure, respectively.The volumetric concentration of the anion plus cation particle shouldexceed 60% and preferably is about 70%.

In all described examples of fixed ion exchange materials 24, there isan intimate fixed, mixture of cation exchanging material and anionexchanging material, and the individual particles or filaments of theexchange materials have a small size. The small size of the ionexchanging particles or filaments and the intimate relationship of thetwo types of exchange resin insures numerous and uniformly distributedsites for water splitting. In all examples (FIGS. 2-10) there also arerelatively large passages for the flow of water when compared to theparticle size, thus providing good water splitting without excessivepressure drop. In particular, the respective ion exchanging materialshave a characteristic dimension that is smaller than the characteristicdimensions of the passages through which the purified water flows. Forthe individual filaments of ion exchanging material, the characteristicdimension is the diameter of the filament. For the individual particlesof ion exchanging material 42, 44, 52, 54, 62, 64, the characteristicdimension is the diameter (or width) of the individual particles. Theflow passages in all examples are not determined by spaces betweenindividual particles or filaments, but instead are determined by thelarger dimensions of the overall strands (for FIGS. 2-7 and 9) or thecombined particles (FIG. 10). As appears from the figures, the flowpassages around the larger structures are substantially larger than thedimensions of the individual particles or filaments. Passages 58 in opencell foam 50 are also substantially larger than the individual cationand anion exchange particles 52, 54 in foam 50. In manufacture, themajority of apparatus 10 is made and assembled the same as for knowndeionization apparatus. Anion and cation exchange filaments 36 and theindividual anion and cation exchange filaments used in braid 38 can bemade from any of the well-known ion exchange materials, e.g., styrenicion exchange resin, acrylic ion exchange resin, phenolic ion exchangeresin, and carbohydrate ion exchange resin. Individual ion exchangeparticles 42, 44, 52, 54, 62 and 64 can be made from the same well-knownmaterials. Binder 44 and binder 64 can be any of the well-knownthermoplastic polymers or thermosetting polymers used in the manufactureof ion permeable membranes. Binder 54 can be any suitable foam materialsuch as polyurethane.

The individual filaments made of either cation exchange resin or anionexchange resin can be made by known techniques, e.g., as described inthe above-referenced patents. The combined exchange particle filament 40and combined particle 60 are made by obtaining (e.g., from acommercially available source) individual anion exchange particles andanion exchange particles in the desired size and approximatelyequinormal amounts, and then applying the binder 46 and 66 using thesame techniques as used for making ion selective membranes, as describedin the above-referenced patents. Open cell foam 50 is similarly made byknown techniques for open cell material, adding the individual anion andcation exchange particles prior to initiating the reaction that createsthe foam.

When using a fabric for fixed ion exchange materials 24, one or morelayers of fabric 28, 30 or 32 is simply placed between membranes 16 and18. When using open cell foam 50, it also is simply placed betweenmembranes 16 and 18. In both cases, there is no need to pack individualion exchange material particles, and there is no need for efforts toobtain uniform packing of particles in the diluting channel. When usingcombined particles 60 for fixed ion exchange materials 24, one packs thediluting channel with particles using the same techniques as presentlyused to pack ion exchange particles except that there is no need toobtain uniformity in the relative amounts of cation exchange particlesand anion exchange particles, because only one type of particle is beingadded.

In operation of deionization apparatus 10, feed and brine are suppliedto diluting channels 20 and concentrating channels 22, respectively, attypical flow rates (e.g., 1 to 3 cm/sec) and pressure (e.g., 5 to 50psig), and electric power is supplied to cathode 12 and anode 14 toprovide an appropriate current density of 2 to 15 mA/square cm andvoltage of 1 to 5 volts per cell pair. The feed supplied to the inletsof diluting channels 20 is typically the permeate from a reverse osmosisprocess. The brine supplied to the inlets of concentrating channels 22is typically a mixture of the reverse osmosis permeate and brinerecirculated from the outlet of the electrodeionization apparatus.

The removal of ions from diluting channels 20 includes two steps. Thefirst step is the diffusion of cations to the cation exchanging solidsand the diffusion of anions to the anion exchanging solids. The secondstep is electrical conduction within the solid phases to the boundingmembranes of the diluting compartment.

The concentration of the ion in question at the surface of the ionexchanging solid is in equilibrium with the concentration of that ion inthe solid. It is desired to increase the exchanging sites havinghydrogen ions and hydroxide ions to increase the transfer of ions fromthe liquid to the solid. The regeneration of exchanging sites withhydrogen ions and hydroxide ions is promoted by water splitting. Thus,by providing numerous, uniformly distributed sites for water splitting(the interfaces between individual ion exchanging particles orfilaments), the transfer of ions to the solid is promoted. Becausetransfer to the solid is the limiting step in the removal of ions, theefficiency of apparatus 10 in removing ions is improved.

The applied electric field then causes anions on the exchanging materialto travel along the anion exchanging material in a conductive path toand through the anion permeable membrane into the concentratingcompartment nearer the anode. The applied electric field similarlycauses cations to travel along the cation exchanging materials in aconductive path to and through the cation permeable membrane into theconcentrating compartment nearer the cathode. The anions and cationsbecome trapped in the concentrating compartments because the movement ofanions toward the anode is blocked by a cation permeable membrane, andthe movement of cations toward the cathode is blocked by an anionpermeable membrane.

By providing numerous, uniformly distributed sites for water splittingin the diluting channels, the removal of ions is improved, and the ionconcentration in the deionized output is advantageously decreasedwithout excessive pressure drops.

Other embodiments of the invention are within the scope of the appendedclaims. For example, fixed ion exchange material could be usefully usedin the concentrating channels 22 as well. Also, a fabric of fixed ionexchange materials 24 could also be made by extrusion.

It is also possible to increase the rate of diffusion of ions from thefluid to the surface of the ion exchanging materials 24, by increasingthe velocity of the fluid relative to ion exchanging materials 24. Thisgoal can be accomplished, without reducing the residence time in theelectrodeionization apparatus, which would counteract the benefits ofthe higher velocity, by two techniques shown in FIGS. 11 and 12. Thefirst (FIG. 11) involves mixing a portion of the water leaving thediluting channel 20 with the feed water entering channel 20 usingrecirculating loop 100. This increases the velocity of the water andalso results in a dilution of the feed water and thus an improved waterpurity. The second technique involves providing a serpentine path whosecross section, normal to the flow direction, is smaller than the crosssection of the electrodeionization compartments. This is achieved byplacing non-permeable obstructions 102 in the flow path so as to createa tortuous path for the flowing water, while keeping the volume of thediluting compartment substantially unchanged.

What is claimed is:
 1. Electrodeionization apparatus for purifying watercomprising a cathode, an anode spaced from said cathode, a plurality ofalternating anion permeable membranes and cation permeable membranesbetween said cathode and anode defining concentrating and diluting flowchannels, each channel being defined between an adjacent pair ofmembranes, the anion permeable membrane defining a diluting channelbeing closer to said anode than said diluting flow channel, the cationpermeable membrane defining a diluting channel being closer to saidcathode than said diluting flow channel, and individual cation exchangeparticles or filaments and individual anion exchange particles orfilaments that are intermixed and fixed in close contacting positionwith respect to each other in said diluting flow channels so as toprovide a plurality of larger, discrete combined structures havingnumerous sites for water splitting at numerous regions of contact ofanion and cation material within each combined structure and provideconductive paths for cations and anions to said adjacent membranes forsaid diluting flow channel and provide flow passages that aresubstantially larger than said particles or filaments for water betweensaid discrete combined structures.
 2. The apparatus of claim 1, whereinsaid individual cation exchange particles or filaments and saidindividual anion exchange particles or filaments are individual cationand anion exchage particles that are fixed together with a binder insufficient particle concentration to provide conductive paths forcations and anions.
 3. The apparatus of claim 2 wherein said combinedstructures are larger combined particles packed into said diluting flowchannel in contacting relation between said adjacent membranes.
 4. Theapparatus of claim 3 wherein the majority of said combined particleshave dimensions greater than 0.1 mm.
 5. The apparatus of claim 4 whereinthe majority of said individual particles of said cation and anionexchange material have dimensions less than 0.01 mm.
 6. The apparatus ofclaim 3 wherein said combined particles are sufficiently large so as tocause an acceptably low pressure drop in said diluting flow channel. 7.The apparatus of claim 6 wherein said pressure drop is less than 100psig.
 8. The apparatus of claim 3 wherein said binder is a memberselected from the group consisting of thermoplastic polymers andthermosetting polymers.
 9. The apparatus of claim 2 wherein saidcombined structures are composite filaments provided as a matrix betweensaid adjacent membranes.
 10. The apparatus of claim 9 wherein saidcomposite filaments are provided as one or more layers of woven ornonwoven cloth between said adjacent membranes.
 11. The apparatus ofclaim 10 wherein said composite filaments have diameters between 0.1 mmand 3.0 mm.
 12. The apparatus of claim 10 wherein said cloth includesgroups of generally parallel composite filaments, with compositefilaments spaced center-to-center by a distance equal to or greater thanthe diameter of composite filaments.
 13. The apparatus of claim 9wherein said binder is a member selected from the group consisting ofthermoplastic polymers and thermosetting polymers.
 14. The apparatus ofclaim 1, wherein said individual cation exchange particles or filamentsare in the form of cation exchange material filaments, and saidindividual anion exchange particles or filaments are in the form ofanion exchange material filaments, and said cation exchange materialfilaments and anion exchange material filaments are combined together inmultifilament strands provided as a matrix between said adjacentmembranes.
 15. The apparatus of claim 14 wherein said multifilamentstrands are braided strands.
 16. The apparatus of claim 14 wherein saidstrands are provided as one or more layers of woven or nonwoven clothbetween said adjacent membranes.
 17. The apparatus of claim 14 whereinsaid filaments have diameters between 0.01 mm and 0.1 mm, and saidstrands have diameters between 1.0 mm and 3.0 mm.
 18. The apparatus ofclaim 14 wherein said matrix includes groups of said strands arrangedgenerally parallel to each other and having a diameter, with strandsspaced center-to-center by a distance equal to or greater than thediameter of strands.
 19. The apparatus of claim 1 wherein said cationexchange particles or filaments are made of a material selected from thegroup consisting of styrenic ion exchange resin, acrylic ion exchangeresin, phenolic ion exchange resin, and carbohydrate ion exchange resin.20. The apparatus of claim 1 wherein said anion exchange particles orfilaments are made of a material selected from the group consisting ofstyrenic ion exchange resin, acrylic ion exchange resin, phenolic ionexchange resin, and carbohydrate ion exchange resin. 21.Electrodeionization apparatus for purifying water comprising a cathode,an anode spaced from said cathode, a plurality of alternating anionpermeable membranes and cation permeable membranes between said cathodeand anode defining concentrating and diluting flow channels, eachchannel being defined between an adjacent pair of membranes, the anionpermeable membrane defining a diluting channel being closer to saidanode than said diluting flow channel, the cation permeable membranedefining a diluting channel being closer to said cathode than saiddiluting flow channel, and cation exchange materials and anion exchangematerials that are fixed in close contacting position with respect toeach other in said diluting flow channels, said materials in each saiddiluting flow channel providing conductive paths to said adjacentmembranes for said diluting flow channel and providing flow passages forwater between said materials, wherein said particles of said cationexchange material and said anion exchange material are intermixed andfixed together in a binder to form an open cell foam having numeroussites for water splitting at numerous regions of contact of anion andcation material between said adjacent membranes, the openings in thefoam being substantially larger than said particles and sufficientlylarge to provide said flow passages with an acceptably low pressure dropin said diluting flow channel.
 22. The apparatus of claim 21 whereinsaid pressure drop is less than 100 psig.
 23. The apparatus of claim 21wherein said binder is a polyurethane foam.
 24. Electrodeionizationapparatus for purifying water comprising a cathode, an anode spaced fromsaid cathode, a plurality of alternating anion permeable membranes andcation permeable membranes between said cathode and anode definingconcentrating and diluting flow channels, each channel being definedbetween an adjacent pair of membranes, the anion permeable membranedefining a diluting channel being closer to said anode than saiddiluting flow channel, the cation permeable membrane defining a dilutingchannel being closer to said cathode than said diluting flow channel,and cation exchange materials and anion exchange materials that arefixed in close contacting position with respect to each other in saiddiluting flow channels, said materials in each said diluting flowchannel providing conductive paths to said adjacent membranes for saiddiluting flow channel and providing flow passages for water between saidmaterials, individual particles of said cation exchange material andsaid anion exchange material being intermixed and fixed together with abinder to form larger combined particles having numerous sites for watersplitting at numerous regions of contact of anion and cation materialwithin each combined particle packed into said diluting flow channel incontacting relation between said adjacent membranes, said cationexchange material and anion exchange material being in sufficientparticle concentration to provide conductive paths, said flow passagesbeing provided in the spaces between and around said larger combinedparticles, said flow passages being substantially larger than saidparticles.
 25. Electrodeionization apparatus for purifying watercomprising a cathode, an anode spaced from said cathode, a plurality ofalternating anion permeable membranes and cation permeable membranesbetween said cathode and anode defining concentrating and diluting flowchannels, each channel being defined between an adjacent pair ofmembranes, the anion permeable membrane defining a diluting channelbeing closer to said anode than said diluting flow channel, the cationpermeable membrane defining a diluting channel being closer to saidcathode than said diluting flow channel, and cation exchange materialsand anion exchange materials that are fixed in close contacting positionwith respect to each other in said diluting flow channels, saidmaterials in each said diluting flow channel providing conductive pathsto said adjacent membranes for said diluting flow channel and providingflow passages for water between said materials, individual particles ofsaid cation exchange material and said anion exchange material beingintermixed and fixed together with a binder to form filaments providedas a matrix having numerous sites for water splitting at numerousregions of contact of anion and cation material within each filamentbetween said adjacent membranes, said cation exchange material and anionexchange material being in sufficient particle concentration to provideconductive paths, said flow passages being provided in the space betweenand around said filaments, said flow passages being substantially largerthan said particles.
 26. Electrodeionization apparatus for purifyingwater comprising a cathode, an anode spaced from said cathode, aplurality of alternating anion permeable membranes and cation permeablemembranes between said cathode and anode defining concentrating anddiluting flow channels, each channel being defined between an adjacentpair of membranes, the anion permeable membrane defining a dilutingchannel being closer to said anode than said diluting flow channel, thecation permeable membrane defining a diluting channel being closer tosaid cathode than said diluting flow channel, and cation exchangematerials and anion exchange materials that are fixed in closecontacting position with respect to each other in said diluting flowchannels, said materials in each said diluting flow channel providingconductive paths to said adjacent membranes for said diluting flowchannel and providing flow passages for water between said materials,said cation exchange materials being in the form of cation exchangematerial filaments, said anion exchange materials are in the form ofanion exchange material filaments, said cation exchange materialfilaments and anion exchange material filaments being intermixed andcombined together in multi-filament strands provided as a matrix havingnumerous sites for water splitting at numerous regions of contact ofanion and cation material within each multi-filament strand between saidadjacent membranes, said flow passages being provided in the spacebetween and around said multi-filament strands, said flow passages beingsubstantially larger than said filaments.
 27. A method of purifyingwater comprising providing a cathode, an anode spaced from said cathode,and a plurality of alternating anion permeable membranes and cationpermeable membranes between said cathode and anode definingconcentrating and diluting flow channels, each said channel beingdefined between an adjacent pair of said membranes, each said dilutingflow channel having individual cation exchange particles or filamentsand individual anion exchange particles or filaments that are intermixedand fixed in close contacting position with respect to each other so asto provide a plurality of larger, discrete combined structures havingnumerous sites for water splitting at numerous regions of contact ofanion and cation material within each combined structure and provideflow passages that are substantially larger than said particles orfilaments for water between said combined structures, the anionpermeable membrane defining a diluting channel being closer to saidanode than said diluting flow channel, the cation permeable membranedefining a diluting channel being closer to said cathode than saiddiluting flow channel, supplying feed water into each said diluting flowchannel, applying a voltage between said anode and said cathode so thatsaid materials in each said diluting flow channel provide conductivepaths for cations and anions removed from said feed water to saidadjacent membranes for said diluting flow channels, removing purifiedwater from said diluting channels, and supplying concentrating waterinto said concentrating channels and removing brine therefrom.